1. Field of the Invention
The present invention generally relates to a sheet folding device, an image forming apparatus, and a sheet folding method.
2. Description of the Related Art
Various types of sheet feeding apparatuses have been proposed which perform folding processing, followed by postprocessing such as punching and stapling, on sheets on which images are formed by a copying machine or other image forming apparatus. Examples of such apparatuses include the known invention described in Japanese Patent Application Laid-open No. 2002-284443. Japanese Patent Application Laid-open No. 2002-284443 describes a sheet folding device which has two folding rollers that are arranged next to and in parallel with each other and rotate in opposite directions to form a fold in a predetermined position of a sheet such as a form, wherein a sheet or a stack of a plurality of sheets is curved and the curved portion of the sheet(s) is taken into between the two folding rollers to form the fold in the sheet(s). The sheet folding device includes: a conveying unit that conveys the sheet(s) so as to pass near adjoining portions of the two folding rollers; a guiding member that changes the course of the leading edge(s) of the sheet(s) so that the sheet(s) conveyed by the conveying unit is/are curved toward the adjoining portions of the folding rollers near the adjoining portions; a stopper that stops movement of the leading edge(s) of the sheet(s) whose moving direction is changed by the guiding member, thereby directing the curved portion of the sheet(s) toward the adjoining portions of the folding rollers; and a folding position restraining member that makes contact with an inner side of the curved portion of the sheet(s) when the sheet(s) is/are curved by the guiding member, thereby increasing the curvature of the curved portion of the sheet(s) before the leading edge(s) of the sheet(s) comes/come into contact with the stopper and bringing the curved portion close to the adjoining portions of the folding rollers to restrain the folding position of the sheet(s).
Related inventions are also described in Japanese Patent Application Laid-open No. 2001-206629 and Japanese Patent Application Laid-open No. 2007-320665.
In the invention described in Japanese Patent Application Laid-open No. 2002-284443, the guiding member is moved by a driving unit regardless of single-sheet folding or multi-sheet folding. The guiding member is thereby brought close to the folding roller nip for the sake of the prevention of folding position errors. According to such a method, single-sheet folding of feeding and folding a single sheet can be successfully performed because the sheet can fall to the position restraining member without fail. In the case of multi-sheet folding where a plurality of sheets are pre-stacked, however, the gap between the guiding member and the sheets may decrease depending on the thickness and curl of the sheets. The smaller gap increases the feeding resistance, and the sheets sometimes fail to fall to the position restraining member. Japanese Patent Application Laid-open No. 2001-206629 and Japanese Patent Application Laid-open No. 2007-320665 include no particular consideration to securing the fall, either.
In the embodiment to be described later, the single feed path corresponds to a second conveying path 17; the conveying unit to the carriage rollers; the restraining unit to a first stopper 23; the guiding member to a movable guide plate 60; the sheet folding device to the reference numeral 5; the driving unit to a pusher member 27, a second motor 57, and a tension spring 61; the control unit to a control unit 56; the elastic body to the tension spring 61; the pusher member to the reference numeral 27; the single driving source to the second motor 57; and the image forming apparatus to an image forming apparatus 1 as a system.